Evidence for an ancient northern ocean comes from candidate shoreline features, from the flatness and youth of the northern plains, and from the deuterium-to-hydrogen ratio in the present atmosphere, which indicates Mars has lost a large fraction of an original water inventory to space. The details remain debated; the broad picture of a much wetter early Mars does not.
Refilling it is a consequence, not a project. Nobody proposes building an ocean directly. An ocean appears if and only if two conditions are met simultaneously and held: surface pressure above the triple point of water, so liquid is stable rather than subliming, and temperatures above 273 K over the basin for a large fraction of the year. Deliver those, and the ice melts on its own and runs downhill.
That makes the ocean the single best progress indicator in terraforming. Every intervention on this site — mirrors, aerosols, gases, polar release — is ultimately measured by whether it can hold water liquid in the northern lowlands. Standing water is the visible proof that the pressure and temperature thresholds have been crossed together.
The realistic version is smaller than the ancient one. Mars has lost much of its original water, and the accessible remainder — polar caps plus mapped subsurface ice — corresponds to a global layer of tens of metres, which pooled into the lowest basins would make substantial seas rather than a hemisphere-spanning ocean. And it would only stay liquid for as long as the warming that produced it is maintained.
The northern lowlands are a ready-made ocean floor kilometres below the highlands. Water released anywhere on Mars flows into it without any engineering.
Standing liquid water is unambiguous proof that pressure and temperature have both crossed their thresholds — a single visible test for the entire warming programme.
A large body of water buffers temperature swings, drives weather and moves heat between latitudes, all of which make the rest of a habitable Mars easier to sustain.
An ocean is a downstream consequence of pressure and temperature, not an independent project. Nothing about it can be attempted before the warming problem is solved.
Deuterium ratios show a large fraction of the original inventory went to space. What remains fills seas, not the ancient ocean.
Liquid water persists only while the forcing is maintained. Stop the aerosols or lose the mirrors and the ocean returns to ice and vapour.
The northern lowlands include some of the most accessible shallow ice and the flattest landing terrain. An ocean puts the early settlement zone underwater.
| Surface pressure | >611 Pa sustained | Above the triple point |
| Temperature | >273 K over the basin | Most of the year |
| Water | Tens of m global layer | Polar caps plus subsurface ice |
| Maintenance | Continuous forcing | It refreezes without it |
Most likely. The northern lowlands form a vast basin kilometres below the southern highlands, candidate shoreline features have been mapped, and the deuterium-to-hydrogen ratio in the atmosphere indicates Mars lost a large original water inventory to space.
Surface pressure above the 611 pascal triple point and temperatures above 273 K over the northern basin, both sustained. Under those conditions existing ice melts and flows into the lowlands on its own.
Smaller than the ancient one. The accessible water — polar caps plus mapped subsurface ice — corresponds to a global layer of tens of metres, which would form substantial seas in the lowest basins rather than a hemispheric ocean.
